Turbochargers for gasoline and diesel internal combustion engines are devices known in the art that are used for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust gas-driven turbine to spin within the housing.
The exhaust gas-driven turbine is mounted onto one end of a shaft that is common to a radial air compressor mounted onto an opposite end of the shaft and housed in a compressor housing. Thus, rotary action of the turbine causes the air compressor to spin within the compressor housing. The spinning action of the air compressor causes intake air to enter the compressor housing and be pressurized or boosted a desired amount before it is mixed with fuel and combusted within the engine combustion chamber.
In a turbocharger, it is often desirable to control the flow of exhaust gas to the turbine to improve the efficiency or operational range of the turbocharger. Variable geometry turbochargers (VGTs) have been configured to address this need. A type of such VGT is one having a variable or adjustable exhaust nozzle, referred to as a variable nozzle turbocharger.
Different configurations of variable nozzles have been employed in variable nozzle turbochargers to control the exhaust gas flow. One approach taken to achieve exhaust gas flow control in such VGTs involves the use of multiple vanes, which can be fixed, pivoting and/or sliding, positioned annularly around the turbine inlet.
The vanes are commonly controlled by a unison ring to alter the throat area of the passages between the vanes, thereby functioning to control the exhaust gas flow into the turbine. The unison ring is disposed within the turbine housing and is rotated by an actuator assembly to move the vanes in a manner to provide the desired control of gas flow. VGTs known in the art make use of a crank arm to translate an actuation movement provided by an actuator to the unison ring. The crank arm is connected to the unison ring by a pin and slot arrangement, wherein the crank arm includes an offset pin that is engaged within a slot in the unison ring. Configured in this manner, actuation of the crank arm causes the offset pin to be moved in a clockwise or counter-clockwise direction, thereby effecting rotational movement of the unison ring in a respective direction.
It is known that VGTs comprising such a crank arm-unison ring arrangement can be prone to suffer from two types of issues. A first issue involves sliding friction and the fact that the crank arm pin that engages the unison ring is know to incur a severe amount of sliding friction, which can cause the pin and/or unison ring slot to wear in an aggressive manner. Such wear can impact the desired interplay of the crank arm and unison ring by not providing the full range of desired movement, or ultimately can cause the crank arm pin to break, thereby prohibiting desired unison ring movement.
A second issue involves the high and cyclic temperatures that the VGT turbine housing and all parts disposed therein are subjected to. In this aggressive temperature environment, the VGT parts are exposed to constant thermal expansion and contraction cycles. Depending on the location, size, and materials used to form the parts that contact one another, i.e., the crank arm pin and unison ring slot, these interconnecting parts may undergo different degrees of thermal expansion and contraction. Such different thermal expansion and contraction characteristics can cause the crank arm pin to bind within the unison ring slot, thereby impairing efficient and dependable unison ring actuation.
It is, therefore, desired that an improved actuation assembly be constructed in a manner that minimizes or eliminates potential impairments to proper vane actuation caused either by unwanted sliding friction or thermal expansion/contraction binding between coupled actuating members. It is desired that such an improved actuation assembly be constructed in a manner capable of providing reliable vane actuation movement after repeated cycles of turbocharger operation. It is further desired that such an improved actuation assembly be configured in a manner permitting retrofit use in existing VGTs without significant modification.